Choosing emission factors you can defend
A hierarchy for Scope 3 factors, and the mistakes that quietly inflate or deflate your inventory.
Your inventory is only as good as its weakest factors — and in Scope 3, the weakest factors usually cover the biggest categories.
Work down the hierarchy
- Supplier-specific data, ideally product-level and verified
- Hybrid — supplier activity data combined with secondary factors
- Average-data — physical quantities times industry-average factors
- Spend-based — economic input-output factors as a last resort
Match the factor to the activity
Check geography, year, unit and system boundary. A cradle-to-gate factor and a cradle-to-grave factor are not interchangeable.
Watch spend-based drift
Spend-based factors move with prices. If inflation lifts your spend, emissions appear to rise with no physical change. Adjust for inflation or move off spend data for large categories.
Document versions
Record the database, version and date for every factor. When a database updates, you'll need to explain the restatement.
Spend effort where it matters
Run a sensitivity check on your top categories. Improving one factor that drives 40% of the total is worth more than polishing ten small ones.